Quote Originally Posted by Photo Engineer View Post

Imagine a paper safe made from red plastic. Paper is not sensitive to red light. Therefore, the safe is 100% light tight as far as paper is concerned but it will fog film.

Of course there are a lot of alternative scenarios for your observation.

Another issue, some plastics degrade over time, so an older plastic paper safe, may still be safe enough for B&W paper, but not for colour paper or film.